power converters and power quality karsten kahle, cern [email protected] cas power converters...

Power Converters and Power Quality

Karsten KAHLE, CERN

[email protected]

CAS Power Converters 2014, Baden (CH) 2

EN 50160 (2010) Voltage characteristics of electricity supplied by public distribution systems

IEC 61000 Electromagnetic compatibility:

IEC 61000-2-2 Compatibility levels for low frequency conducted disturbances and signalling in public low voltage (LV) power supply systems

IEC 61000-2-4 Compatibility levels in industrial plants for low-frequency conducted disturbances

IEC 61000-2-12 Compatibility levels for low frequency conducted disturbances and signalling in public medium voltage (MV) power supply systems

IEC 61000-3-4 Limitations of emissions of harmonic currents in LV power supply systems for equipment rated > 16A

IEC 61000-3-6 Assessment of emission limits for distorting loads in MV and HV power systems

IEC 61000-4-7 General guide on harmonics and interharmonics measurements ....for power supply systems and equipment connected thereto

VEÖ- VSE- CSRES-VDE Technical rules for the assessment of public power supply compatibilities (in German); VEÖ - Verband der Elektrizitätswerke Österreichs, VSE -Verband Schweizerischer Elektrizitätswerke, CSRES – Ceske sdruzeni regulovanych elektroenergetickych spolecnosti, Forum Netztechnik im VDE (2007) and technical annex document (2012)

CAS 2004, Warrington Electrical Network and Power Converters, H. U. Boksberger, PSI

CERN, ref. EDMS 113154 Main Parameters of the LHC 400/230 V Distribution System https://edms.cern.ch/file/113154/2/LHC-EM-ES-0001-00-20.pdf

References

3CAS Power Converters 2014, Baden (CH)

What is Power Quality? Classification of disturbances Statistics (example CERN) Additional power quality considerations

Electrical networks and pulsating power Systems without energy storage Systems with integrated energy storage

Conclusions

CAS Power Converters 2014, Baden (CH)


4



Conclusions

CAS Power Converters 2014


5

VOLTAGE DIP / VOLTAGE SWELL

-1500

-1000

-500

0

500

1000

1500

0 50 100 150 200 250 300

time (ms)

volt

age

(V)

MAINS FAILURE

-800

-600

-400

-200

0

200

400

600

800

0 50 100 150 200 250 300

time (ms)

volt

age

(V)

HARMONICS

-1000

-500

0

500

1000

1500

0 10 20 30 40 50 60

time (ms)

volt

age

(V)

TRANSIENTS 900 V for 0.1 ms

-1000

-500

0

500

1000

1500

0 10 20 30 40 50 60

time (ms)

volt

age

(V)

DIP SWELL

Classification of disturbances

6CAS Power Converters 2014

VOLTAGE DIP / VOLTAGE SWELL

-1500

-1000

-500

0

500

1000

1500

0 50 100 150 200 250 300

time (ms)

volt

age

(V)

HARMONICS

-1000

-500

0

500

1000

1500

0 10 20 30 40 50 60

time (ms)

volt

age

(V)


-1000

-500

0

500

1000

1500

0 10 20 30 40 50 60

time (ms)

volt

age

(V)

MAINS FAILURES

Causes:- thunder-storms- short-circuits inside CERN- Emergency Stop operation

Consequences:- accelerator stop

DIP SWELL



HARMONICS

-1000

-500

0

500

1000

1500

0 10 20 30 40 50 60

time (ms)

volt

age

(V)


-1000

-500

0

500

1000

1500

0 10 20 30 40 50 60

time (ms)

volt

age

(V)

Causes:- sudden change of load, inrush- short-circuits inside & outside CERN- thunder-storms

VOLTAGE DIP / SWELL

Consequences:- sometimes accelerator stop

MAINS FAILURES

Causes:- thunder-storms- short-circuits inside CERN- Emergency Stop operation




HARMONICS

-1000

-500

0

500

1000

1500

0 10 20 30 40 50 60

time (ms)

volt

age

(V)

TRANSIENTS

Consequences:- failure of electronics

Causes:- switching capacitor banks ON (SVC’s)- thunder-storms

MAINS FAILURES

Causes:- thunder-storms- short-circuit inside CERN- Emergency Stop operation



VOLTAGE DIP / SWELL




HARMONICS

Causes:- non-linear loads (power converters, computer centers, PC’s)

Consequences:- malfunctioning of electronics- overload of Neutral conductor

MAINS FAILURES

Causes:- thunder-storms- short-circuit inside CERN- Emergency Stop operation



VOLTAGE DIP / SWELL


TRANSIENTS

Consequences:- failure of electronics

Causes:- switching capacitor banks ON (SVC’s)- thunder-storms





Conclusions



voltage disturbances

020406080

100120140160180200

0 100 200 300 400 500 600 700 800 900 1000

duration (ms)

amp

litu

de

(%)

100% = nominal voltage

red colour: major events (accelerator stop)

…. long-term

Power quality statistics (CERN network)

Voltage disturbances 400 kV


voltage disturbances

020406080

100120140160180200

0 100 200 300 400 500 600 700 800 900 1000

duration (ms)

ampl

itude

(%)



…. long-term


Voltage disturbances 18 kV


voltage disturbances (2003)

020406080

100120140160180200

0 100 200 300 400 500 600 700 800 900 1000

duration (ms)

ampl

itude

(%)



…. long-term


Voltage disturbances 400 V


Total Harmonic Distortion

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

EB

D1

/2R

EB

D1

/2E

ER

D1

/2R

EB

D1

/25

EZ

D1

/25

EX

D1

/25

ES

D3

/25

EO

D1

/2X

EX

D1

/2X

ER

D1

/4R

ES

D3

/45

Har

mo

nic

Dis

tort

ion

TH

D (

%)

IEC 61000-2-2, class 2

Power quality statistics (CERN 400 V networks)

UP

S

IEC 61000-2-2, class 1


The MAJORITY of power quality issues is caused inside CERN.

The MAJORITY of network disturbances has no consequences.

Power quality statistics




Conclusions



voltage disturbances (2003)

020406080

100120140160180200

0 100 200 300 400 500 600 700 800 900 1000

duration (ms)

ampl

itude

(%)

Before constructing the LHC, CERN specified the immunity levels for all electrical equipment.This internal standard intends to assure a certain minimum immunity of equipment, with the objective to significantly increase MTBF of the LHC.

Unfortunately, it shows now during LHC operation, which equipment does not sufficiently respect this standard. In particular, voltage dips and voltage swells remain the main power quality issues for LHC.

Specification of immunity of electrical equipment

https://edms.cern.ch/file/113154/2/LHC-EM-ES-0001-00-20.pdf

“Main Parameters of the LHC 400/230 V Distribution System”

Nominal voltage 400 / 230 V

Max. voltage variations ± 10 %

Typical voltage variations ± 5 %

Transients (spikes) 1200 V for 0.2 ms

Voltage swells + 50 % of Un, 10 ms

Voltage dips - 50 % of Un, 100 ms

Total harmonic distortion (THD) 5%

Standardised CERN immunity levels

for voltage variations

… long-term

Typical disturbances


Propagation of external disturbances into a distribution network

Propagation of external asymmetrical disturbances into a network depends on the combination of transformer vector groups (e.g. 400 kV voltage dips going into CERN network):

R S T R-S S-T R-T400 kV 50 % 100 % 100 % 75 % 100 % 75 %66 kV 58 % 97 % 96 % 78 % 100 % 77 %18 kV 77 % 100 % 77 % 95 % 96 % 65 %

18/0.4 0.4 kV 94 % 94 % 66 % 100 % 77 % 77 %

18/3.3/0.43.3 kV 94 % 94 % 66 % 100 % 75 % 78 %0.4 kV 94 % 94 % 66 % 100 % 76 % 76 %

R S T R-S S-T R-T400 kV 50 % 97 % 50 % 76 % 76 % 50 %66 kV 57 % 87 % 58 % 76 % 76 % 50 %18 kV 76 % 76 % 50 % 83 % 60 % 60 %

18/0.4 0.4 kV 83 % 60 % 60 % 77 % 50 % 77 %

18/3.3/0.43.3 kV 84 % 66 % 64 % 77 % 53 % 77 %0.4 kV 83 % 65 % 65 % 78 % 54 % 78 %

voltage level faulty phase400 kV R66 kV R18 kV R-T3.3 kV T0.4 kV T

voltage level healthy phase400 kV S66 kV S18 kV R-S3.3 kV R0.4 kV R

Principle of propagation

Double-phase dipSingle-phase dip

Double-phase dip (-50% in phases R and T, healthy phase = S)

Single phase dip, -50% in phase R


Flicker: Impression of unsteadiness of visual sensation induced by a light stimulus whose luminance … fluctuates with time.

Voltage fluctuation: Changes of r.m.s. voltage evaluated as a single value for each successive halfperiod of the source voltage

Short-term flicker indicator Pst: Flicker severity evaluated over a short period (in minutes); Pst = 1 is the conventional threshold of irritability

Long-term flicker indicator Plt: Flicker severity evaluated over a long period (a few hours) using successive Pst values

Flicker

Flicker caused by power converters for accelerators

Some thoughts:- Flicker limits are based on the empiric definitions of the human eye’s sensitivity to luminance fluctuations.- Flicker limits are important contractual parameters at the point of connection to the external grid, to be strictly respected!- Inside the physics laboratory, the irritating effects of flicker can be reduced by strictly separating general services (lighting)

and power converter networks.

Ref. IEC 61000-3-3 fig. 4

Number of voltage changes per minute

Note: 1200 voltage changes per minute = 10 Hz flicker

rela

tive

volta

ge c

hang

e Δ

U/U


Effects of pulsating loads on external networks

Example:

CERN SPS

- Pulsating reactive power is compensated within CERN (SVC’s)

- Pulsating active power is supplied by the 400 kV network

Δ U (400 kV) due to SPS: < 0.6 % pk-pk *)

Δ f (400 kV) due to SPS: 5 … 25 mHz pk-pk *)

*) depending on 400 kV network configuration (and its Scc)

External 400 kV network

In general:

Reactive power variations cause voltage variations (flicker)

Active power variations cause frequency variations in the grid

400 kV network

130 kV network


230 MW peak

Class 1: Protected supplies for compatibility levels lower than those on public networks … for very sensitive equipment.

Class 2: Environments of industrial and other non-public power supplies … and generally identical to public networks.

Class 3: Industrial environments, in particular when– a major part of the load is fed through converters; -> Hey, that’s a particle accelerator!– loads vary rapidly. -> Yes, a particle accelerator!– ….

Power converters for particle accelerators represent the roughest type of load, comparable to heavy industry such as large arc furnaces, rolling mills etc. (class 3).

However, to operate them correctly and with the required precision, power converters for particle accelerators require compatibility levels sometimes better than the most sensitive equipment (class 1).

Electromagnetic environment classesacc. IEC 61000-2-4

What do these three classes actually mean?

Class 1 Class 2 Class 3 CERN Engineering Spec. Example: SVC for SPS (18 kV)

Voltage tolerances ± 8% ± 10% -15% / +10% typically ± 5%, max. ± 10% ± 0.75% (transient)

THD(400V) 5% (short-term 7.5%) 8% 10% (short-term 15%) typically 2%, max. 5% 0.75% (transient)

Frequency tolerances ± 1 Hz ± 1 Hz ± 1 Hz ± 0.5 Hz ± 0.5 Hz




Conclusions



*) FACTS = Flexible AC Transmission Systems

Power quality improvement by SVC’s *)

Building for thyristor valve, cooling and control room

Thyristor controlled reactors

Harmonic filters

Example: SVC for SPS (TCR 150 Mvar, -130 Mvar harmonic filters)


Capacitor banks (=harmonic filters)

- constant Mvar generation, p.f. ≠ 1

- constant voltage support (constant voltage increase)

- harmonic filtering

- always requires tuning to control resonances!

Static Var Compensators (SVC’s)

- variable Mvar generation -> p.f. ~ 1

- variable voltage support (stabilisation Uref)

- harmonic filtering

harmonic

filters

Thyristor controlled reactors (TCR)

harmonic

filters

Power quality improvement by SVC’s *)

*) FACTS = Flexible AC Transmission Systems


a) Reactive power compensation

Without SVC:

- load reactive power taken from the network

- transmission and distribution system

needs to be rated for apparent power S

- reactive power variations cause flicker

- contractual power factor at grid connection point

- reactive power consumption costs money!

S’

Q

P

-QSVCSQ

P

With SVC:

- load reactive power is compensated locally

- lean transmission and distribution system

- reduced transmission losses

- disturbing effects of pulsating reactive and

active power eliminated (no flicker)

Thyristor power converters consume (pulsating) active and reactive power.


F2 F3 F5TCR F7 F11 F13 HF1 HF2Pulsating

TransformerEHT2

EMD2/BE 18 kV

Load

reactive power

50% SPS

TCR

150 Mvar

filters

-130 Mvar

SPS power converters

90 Mvar



Active and reactive power


Reactive power taken from EDF is almost zero!

Reactive power consumed by load (SPS)

Reactive power generated by SVC

Active power consumed by load (SPS)

Example: SVC BEQ3 for SPS


b) Voltage stabilisation

- during each power pulse, the voltage of the network decreases

- periodic power cycling causes unwanted periodic voltage drops (flicker)

- the principal cause: changing Mvars flowing through the inductance of the power network

Network 400kV

source load

Voltage drop 1 + Voltage drop 2 + Voltage drop 3

U = RI cos + XI sin U

Transformer 400/18 kV Cables 18 kV

R + jX R + jX R + jX

This term is the principal responsible for voltage variations due to (pulsating) reactive power of the load:

Inductive load -> voltage drop Capacitive load -> voltage increase!



How to keep the network voltage constant during the power pulses?

Solution: SVC generates a specific pulse of reactive power, to compensate for the unwanted drops caused by pulsating active and reactive power of the load.

• •

• With of the supplying network

reactive power generated by SVCactive power consumed by loadreactive power consumed by loadnetwork short-circuit power

Nota: An SVC cannot assure perfect Mvar compensation and perfect voltage stabilisation at the same time. We need to allow for small variations of reactive power to correct the disturbing effects of active power variations.



How to vary the reactive power generation of an SVC?

Variant 1: Variation in discrete steps

Variant 2: Capacitor bank and TCR(continuous variation between 0 and max.)

C1 C2 C3 Capacitance step size, example: 22 21 20

mechanical switches

or thyristors

thyristor valve

(bi-directional)

thyristor controlled reactor (TCR)

capacitor bank

(always ON)


How can an SVC generate a pulse of (capacitive) reactive power?

Let’s take variant 2 from previous page: Capacitors and TCR(continuous variation between 0 and max. reactive power generation)

C TCR

QC

QTCR

QSVC = QC + QTCR


Typically, an SVC should stabilise the network voltage to ±1%, even for fast load changes


Limitations of SVC technology:- Response time 50-100 ms, hence unsuitable for correction of fast transient network disturbances- Mvar output decreases with network voltage, hence unsuitable for voltage support at low system voltage

𝑄=3∗𝑈 2∗ω∗𝐶

18 kV network voltage+/- 0.75 %+/- 0.3 %

0 5 10 150.98

0.985

0.99

0.995

1

1.005

1.01

1.015

1.02Phase-to-phase RMS voltage

[p.u

.]

Time [s]

18 kV voltage response


Typical SVC response time: 50–100ms

Example SVC for SPSVoltage variations @18kV:Without SVC: -14%With SVC: ±0.75%


c) Harmonic filtering

Feeder from 18 kV substation

Example: SVC for SPS

Measured harmonic distortion

0

0.1

0.2

0.3

0.4

0.5

0.6

50

100

150

250

350

550

650

850

950

1150

1250

frequency (Hz)h

arm

on

ic d

isto

rtio

n (

%)

Measured harmonic distortion

0

0.1

0.2

0.3

0.4

0.5

0.6

50

100

150

250

350

550

650

850

950

1150

1250

frequency (Hz)h

arm

on

ic d

isto

rtio

n (

%)

𝑓 𝑟𝑒𝑠=1

2𝜋 √𝐿𝐶First parallel resonance with network (107 Hz) is mastered by 100 Hz and

150 Hz filters

Tuned to typical harmonic frequencies of 6p and 12p

thyristor converters (250, 350, 550, 650 Hz)

Damped high-pass filters 950 Hz and 1050 Hz

1050 Hz filter can be disconnected to operate

SVC with -112 Mvar instead of -130 Mvar

(reduced SVC losses)

SVC split in 3 groups due to circuit breaker limits for

capacitive switching

TCR rated 150 Mvar (filters -130 Mvar) to allow 20 Mvar

inductive SVC output in case of high network system voltage

Harmonic performance:indiv. harm. max. 0.5 %

THD(18 kV) max. 0.75 %

Comparison: Limit = 8% for IEC 61000-2-2 class 2


Summary of SVC performance

Example: SVC for SPS


The reactive power values in the first line concern one system (50% of SPS). For total SPS, multiply by 2.

Most common harmonic filter configurations for SVC’s

C-type filter

- reduced 50 Hz losses

- requires add. type of C

- typically for 100 and 150 Hz

L-C-type filter

- for 250, 350, 550 and 650 Hz

High-pass filter

- 850 Hz and above

In all configurations, the capacitor banks are in double-star connection, star-points not connected to earth.

Star connection is preferred to delta, for better capacitor protection, and to limit the need for series connection of capacitor units.

Har

mon

ic c

urre

nt

50 H

z


TCR configuration for SVC’s

Losses of SVC technology:

- at zero Mvar output, the TCR current cancels out the capacitive current of harmonic filters (unnecess. losses!)

- overall, the relative losses of an SVC are quite small:

harmonic filters = 0.1%

TCR + thyristors = 0.4%

(of total Mvar rating)

Delta connection of reactors:

- to trap the triple harmonics in the delta (reduced harmonics from the network)

- it’s cheaper to build thyristor valves with high voltage than high current (thyristor series connection)

- in unearthed star connection, the starpoint would move

Typical TCR control strategy

Thyristor controlled reactors (HV part)


Design considerations for SVC’s for particle accelerators

1. Expected performance?

- Controlling DC magnet current to ppm precision (and minimise ripple), requires clean AC supply!

2. Voltage level

- Typically, SVC’s are connected to MV network 10 – 36 kV.

- Var compensation at LV level 400 V not recommended (changing network configurations, resonances …)

3. Choice of technology

- Harmonic filters (could be switched in groups, depending on load situation), with thyristors or switches

- SVC: Harmonic filters, combined with thyristor controlled reactors, compensation of reactive power

- STATCOM: compensation of active and reactive power

4. Electrical location

- Where is the optimum connection point for best SVC performance?

5. Rating

- Minimum Mvar rating of harmonic filters and TCR: to compensate for voltage variations due to Q and P

- TCR and harmonic filters do not need to have identical Mvar rating; SVC could also be asymmetrical

6. Harmonic filter design

- Typical spectrum of power converters: n*p ±1, with n = 1, 2, 3 and p = 6 or 12-pulse -> F5, F7, F11, F13 and HF filters

- Connecting capacitor banks: parallel resonance with network: , then F2 or F3 might be required


TCR reactors (50 Mvar / ph)

SVC – how does it look like?

Thyristor valve 18 kV, 2800 A Filter reactors

Filter capacitors Cooling plant for thyristor valve Harmonic filter protection




Conclusions



Decoupling of cycling pulses of active and reactive power from the network (e.g. BNL and CERN)

Rotating machines

AC

AC

AC

DC

6 kV AC

18 kV AC

7 MVA

3 * 195 V AC

0.8 MVA

190 V AC

460 kVA

2.5 kV AC

6.6 kV AC

Mass of 95 Tons at 1000 rpm kinetic energy = 233 MJ

90MVA6MW 6 kA / 9 kV

Motor Generator

2 * 12 MVA

2 * 12 MVA

Ma

gne

ts

BNL-AGS

courtesy: I. Marneris

CERN-PS

Decoupling:From network: around 6 MW peakThe load: 45 MW / 65 Mvar peak

Parameters (MPS CERN):Generator power: 95 MVA peakStored energy: 233 MJ @1000 rpmSpeed variation: 48 Hz - 52 Hz


Example: Decoupling of power converters from external network disturbances (ESRF)

Rotating machines

Conditionning zone

Disconnection zone

Two twin rotablocs (4 accumulators and 4 alternators in one cell)

External voltage drops 20 kV >│-10%│

- Conditioning zone: The alternators permanently compensate for the poor power quality.

- Disconnection zone: The system isolates the incoming power and fully compensates for the drop.

Summary: - ESRF is surrounded by 3 mountain chains. Many thunderstorms in summer. - stored energy 100 MJ, can compensate during 3 s for 100% of missing power. - significant power quality improvement during operation, and reduction of accelerator stops and downtime. MTBF for X-ray production increased 24h -> 60h.


All information on this slide: Courtesy of J.-F. Bouteille, ESRF Grenoble

Example: Decoupling of power power pulses from the network (POPS – Power System for PS)

Power converter with integrated energy storage

flying capacitors

DC/DC converters transfer the power from storage capacitors to magnets.Four flying capacitors banks are charged via the magnets, and not connected to the mains. Only two AC/DC converters (AFE) supply the losses of the system+magnets from the mains.

AC/DC chargers (AFE)

DC/DC converters

DC3

DC

DC +

-

DC1

DC

DC

DC5

DC

DC +

-

DC4

DC

DC-

+

DC2

DC

DC-

+

DC6

DC

DC

MAGNETS

+

-

+

-

CF11

CF12

CF1

CF21

CF22

CF2

AC

CC1

DC

MV7308

AC AC

CC2

DC

MV7308

AC

18KV ACScc=600MVA

OF1 OF2

RF1 RF2

TW2

Crwb2

TW1

Lw1

Crwb1

Lw2

MAGNETS

AC/DC converter - AFE

DC/DC converter - charger module

DC/DC converter - flying module

magnets

Patent: European Patent Office, Appl. Nr: 06012385.8 (CERN & EPFL)

60 MW6 kA / ±10 kV


Magnet current and voltage Voltage and current of the magnets

-10000

-8000

-6000

-4000

-2000

0

2000

4000

6000

8000

10000

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Temps [s]

U [V]

I [A]

Power to the magnets

Stored energy in magnets

Capacitor voltagePower from the mains =Magnet resistive losses

Active power of the magnets

-60000000

-40000000

-20000000

0

20000000

40000000

60000000

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Time [s]

Po

wer

[W

]

.

+50MW peak

Capacitors banks voltage

0

1000

2000

3000

4000

5000

6000

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Time [s]

Vo

ltag

e [V

]

.

5kV to 2kV

Inductive Stored Energy of the magnets [J]

0

2000000

4000000

6000000

8000000

10000000

12000000

14000000

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Time [s]

En

erg

y [J

]

.

12MJ

Resistive Losses and charger power

0

2000000

4000000

6000000

8000000

10000000

12000000

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Time [s]

Po

wer

[W

] .

Losses

10MW

Energy management of POPS


Energy storage capacitor banks (x6)


POPS 6kA/±10kV

Capacitor banksPower converter room Cooling towerControl room Power transformers





Conclusions



Excellent power quality is essential to make excellent physics!

The most important recommendations and conclusions from my talk are:

- If the IEC 61000 does not cover sufficiently the specific needs of your physics laboratory, you need to define the principal power quality standards for your electrical equipment!

- All groups installing and operating electrical equipment need to be involved in power quality considerations, from the beginning.

- Strictly separate (pulsating) power converter loads from general services loads (supply via different transformers).

- Minimise network impedances (inductances!) to reduce voltage variations and harmonic distortion in your networks.

- When choosing a power converter topology, aim to minimise the amplitude of pulsating reactive and active power.

Conclusions


power converters and power quality karsten kahle, cern [email protected] cas power converters...

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